The present invention relates to an echo sounder, particularly for locating fish, of the type employing a cathode-ray tube employing a raster scanning pattern to display echo signals.
A known echo sounder of this type includes a main memory composed of series-connected shift registers whose number corresponds to the number of scanning lines, with the memory capacity of each register corresponding to the number of image points or elements, on each line so that, as a whole, a single picture for the screen of a cathode-ray tube display can be stored. The output of the last shift register of the series arrangement is connected to a color converter of the display and the input of the first shift register is connected, via a gating circuit, with a data read memory which also is a shift register having the same memory capacity as one shift register of the main memory. Echoes received from a receiving transducer during one sounding period are read into the data read memory and at the same time echoes already stored in the data read memory are transferred to the main memory. The data read memory stores representations of all echoes received during one sounding period.
To display the memory contents of the main memory on the screen of the display device, the output information of the last stage of the last shift register in the main memory is fed to the color converter and, via the gating circuit, is simultaneously fed back to the first stage of the first shift register in the main memory. Since the circulating period for the memory contents is equal to one picture scanning period, the memory contents are displayed on the display device in the form of a still picture. In addition, the output of the last shift register in the main memory is connected, via a delay shift register having the same memory capacity as one shift register in the principal memory and via the gating circuit, to the input of the first shift register in the main memory.
Each time the data are transferred from the data read memory to the principal memory, the information from the immediately preceding sounding period becomes visible in a vertical line at one edge of the screen of the display device and the oldest previously stored sounding period information is eliminated at the other edge of the screen. The data relating to the intervening sounding periods become visible in vertical rows which are offset toward the outer edge of the screen corresponding to their age. Thus the display is the same as that recorded on a paper chart in a conventional fish finding echo sounder and the display moves in the same direction, here from the right to the left on the screen, as in the conventional fish finding echo sounder in which the recording paper is moved in the opposite direction, i.e. from the left to the right.
The above-described known fish finding echo sounder has the advantage over the known mechanical echographs of not requiring chart paper, so that no consumable material is required any longer, and because of its relatively low susceptibility to malfunction. However, in addition to the relatively large amount of circuitry involved, one other decisive drawback must be accepted. The recording on the screen of the display takes place in a time offset due to the data read memory involved, i.e. out of sync with the echoes arriving from the sounding. This makes parallel evaluation of the sounding in an additional listening channel extremely difficult and then only with considerable, hardly justifiable additional expenditures. Such additional echo evaluation with listening channel is still necessary, however, and is of considerable advantage with a low signal to noise ratio in the received echoes.
Although it is theoretically possible to omit the data read memory and to read the echoes received during one sounding period directly into the principal memory, which would provide an almost time synchronous display of the sounding, in practice this is possible only for the unique case where the scanning rate of the cathode-ray tube display device and the input rate of the echoes received from the receiving transducer are identical.
If one assumes a horizontal resolution of the cathode-ray tube display of, for example 512 vertical rows, for a repetition rate of the display of 50 Hz, this results in a sounding duration or sounding period for a single sounding of 1/(50.multidot.512)=39 .mu.s. Given the speed of sound in water of 1500 m/s, this would correspond to a maximum sounding depth for the fish finding echo sounder of 0.5.multidot.1500.multidot.39=29 mm. Such a fish finding echo sounder, however is completely useless, so that the data read memory is an absolutely indispensable component of the known fish finding echo sounder.
For reasons of circuit engineering, in the known fish finding echo sounder the present sounding period is always stored in the data read memory until a further sounding is being made and is only then transferred in the main memory and displayed on the display device thus preventing time synchronism between the actually received echoes, which can be made audible through a loudspeaker after passing through a frequency converter, and the display on the screen of the display device. On the latter, the echoes are displayed one sounding period later, i.e. with a sounding depth of e.g. 2000 m about 2.7 s later, at a time when in the listening channel the echoes of the next following sounding are already audible. Comparison between the audible echoes and the visible echoes is thus completely impossible.